1. Field of the Invention
This invention relates generally to background generation in video photography. More specifically, this invention relates to chroma replacement technology in background removal and subsequent replacement with video and digital imaging.
2. Description of the Related Art
The ability to replace a scene's background allows the television and film industries the freedom to create scenes not otherwise possible, whether due to the actual background's location or the prohibitive expense associated with creating the desired background. By using chroma replacement technology, however, it is possible to generate a wide variety of backgrounds in the studio. This technology allows a photographer to shoot scenes in front of a solid colored background, digitally remove the background from the film, and replace it with a new background image. This technology is found in motion pictures, videos, and select novelty type portraiture (heads on different bodies, etc.), but has not typically been useful in the creation of high quality professional portrait or commercial prints.
FIGS. 1A and 1B illustrate a typical approach to background replacement in the prior art. The chroma replacement (also known as blue or green screen or chroma keying) approach to creating backgrounds for video, commercial, and digital imaging involves placing a colored background 10 behind a subject 12, where the background is typically a blue or green colored paper. Multiple flood lamps 14 generally illuminate the colored background 10 from several directions. After taking the picture of the subject using film studio cameras 16, technicians separate the resulting film image to electronically remove the colored background and replace it with a pre-selected image. The modified image is suitable for printing with a digital printer or is reconverted to a negative using a film recorder and printed with traditional negative printing techniques. In video photography, such as television, the replacement process typically occurs in real time by using an electronic filter to remove on the fly the background color and either simultaneously or downstream merge a new background into the viewing image.
The limitations associated with the current chroma replacement procedure in film and video photography are numerous. For example, the colored background image may have significant uneven illumination unless precise proper positioning of numerous flood lamps occurs. The uneven illumination makes it difficult to electronically remove the background color before replacing it with the desired background image due to the large bandwidth of the color to be replaced. An additional problem is that the flood light illumination on the green or blue background often results in a large amount of color distortion on the subject caused by the intense colored light reflecting from the background onto the subject.
Another limitation with chroma replacement technology in film and video photography is that the subject may cast shadows onto the background screen that become distractions to the viewer. The shadows cast onto the background intensify as the subject stands closer to the background and make it more difficult to successfully remove the background. An example of this occurs in television when the weatherman stands close to the background being electronically converted into the weather map and his shadow is visible on the background.
The commercial still photography industry does not typically use chroma replacement due to several limitations in the technology. One important limitation is that the color of the background limits the colors used in the foreground of the picture. If the subject wears or holds anything of similar color to the background color, the chroma replacement process will replace both the background colored image and the similarly colored item. It is therefore necessary to broadly avoid colors in the foreground that have even a similar color to the background color. For example, if using a blue screen background, the subject cannot wear any shade of a blue shirt or a blue tie. Similarly, if the photographer uses a green screen, green plants cannot be in the scene's foreground. In television, the subject is often told in advance what colors to wear to avoid this problem of having a color similar to the color of the background. However, this is a common problem in a photography studio where the photographer has no advance warning and little control over the colors the unaware subject chooses to wear to the studio.
Since chroma replacement has not been a viable option for commercial still photography, still photographers typically use composite photography instead. In composite photography, a projector projects an image at a blank screen behind the subject. In this manner, a different background can be created within the studio. The front projection process traditionally used to create composite photographs in still photography has many limitations in its use. A front projection device typically comprises a large heavy projector with an attached beamsplitter that mounts to a tripod or stand. A camera then securely mounts to the front projection device (on the stand) and can be brought into alignment with the front projection device by raising and lowering the device in relation to the camera by adjusting the tilt of the beamsplitter and the pivot of the camera. This process often leaves fringe shadowing around the subject since the adjustments are rarely done with enough precision to achieve perfect alignment. Additionally, if the photographer wants to change the camera from a vertical to a horizontal format or vice versa, it is necessary to spin the camera on the mounting blocks (of the stand) while the front projection device remains stationary. Changing the camera causes the camera to go out of alignment with the front projection device, which means that the photographer must precisely realign the camera with the front projection device.
The present invention involves a novel application of the front projection process to the chroma replacement technology for the film and television industries that solves many of the problems currently associated with the replacement technology. The present invention comprises a retroreflective background screen and a unique lightweight projector and beamsplitter combination that forms a front projection device. This front projection device quickly attaches to any camera of any size and automatically aligns the projection beam with the exact center of the camera lens. The small, simple attachment with its precise positioning makes it a highly unique instrument offering many advantages over prior techniques.
The present invention improves the chroma replacement process in film and video photography by eliminating the need for a specific colored background and flood light illumination of the background. Eliminating the flood lamps solves problems such as uneven illumination of the background, color contamination of the subject caused by reflection of the colored light from the background onto the subject, and shadowing from the subject on the background. Elimination of these problems allows the subject to be positioned as close as wanted to the background. With the present invention, the subject will not cast visible shadows onto the background screen regardless of how close to the background the subject stands because the subject's body precisely fits within its shadow, making the shadow invisible to the camera.
The present invention also eliminates the need for multiple backgrounds of different colors to be kept by a film, television, or commercial photography studio. With a retroreflective screen, the background can take on any conceivable color simply by inserting a different colored filter into the present invention. By changing the colored filter, the color of the background quickly switches to a color maximally compatible with the chroma replacement process. Thus, a background of any number of colors is available with only one screen required.
The present invention additionally improves the current front projection process used in commercial photography. To begin with, the automatic alignment feature of the invention eliminates the time consuming adjustment of the camera, beamsplitter and projector previously required in the front projection process. This feature eliminates fringe shadows around the subject and creates maximum color saturation in the background. Also, as the photographer shifts the camera's orientation from horizontal to vertical or vice versa, the present invention turns precisely on the camera lens and maintains perfect alignment with the projector.